Nanostructured metal surfaces currently play an important role in electronics, optics, biodiagnostics and catalysis (1-3—see reference listing hereinafter). Similar to a photoresist in conventional photolithography, self-assembled monolayers (SAMs) of alkanethiols have been utilized as masks to direct the patterning of underlying thin metal films. Such a patterning has relied on at least two approaches. A first approach is an indirect approach, in which a focused ion (4) or electron beam (5-8), photoradiation (9-10) or a scanning probe microscope tip (11) locally excites or degrades an adsorbed monolayer. In most cases, the monolayer is damaged or destroyed (12-13), but in some cases, the monolayer's cross-linking is effected (14). SAMs patterned using the indirect approach have been utilized as masks to control the electrodeposition of metal salts or substrate etching at the exposed regions of thin metal films (5, 7, 15-17). Still, the indirect patterning of alkanethiols is a low-throughput process that typically requires serial scanning with an exception of photolithographic methods with limited patterning resolution.
A second approach of alkanethiol patterning uses direct deposition tools, such as micro-contact printing (μCP) (18) or dip-pen nanolithography (DPN) printing (19, 20) to directly deposit alkanethiols onto a thin metal film. μCP and its variants allow for massively parallel printing organic compounds on surfaces while controlling feature size typically down to about 200 nm. μCP generated patterns of hexadecanethiol (CH3(CH2)15SH) have been used as a mask for the electroless deposition of Ni on Au (3). μCP patterned alkanethiols of various lengths can be used to direct the electrodeposition of Ag and Au salts depending on the applied potential (21).
DPN printing is a direct-write tool for tailoring the chemical composition of surfaces on the sub-50 nm to many micrometer length scale (19, 20). Small organic molecules (20, 22-24), oligonucleotides (25), proteins (26), conducting polymers (27) and sol gels (28) have been patterned on inorganic substrates, such as Au, Ag, and SiOx, using DPN printing. DPN-patterned alkanethiols can be used as negative chemical etch resists for generating a variety of inorganic nanostructures. For example, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) were each used as chemical etch resists to generate 12 nm gaps and sub-50 nm metal disks comprising Au, Ag, or Pt on a silicon substrate (29-33).
Approaches using alkanethiols as a positive etch resist have been proposed and demonstrated for μCP using bulky or poorly ordered adsorbates that provide high adsorbate solution exchange resistance, but are poor etch resists (34, 35).